Solid state gamma and X-ray detectors are used for many applications which require precise spectroscopic measurements. These applications include security, medical, space and astrophysical research, reactor safety, and a host of others. Room temperature operation is a very important consideration in many of these applications. A gamma or X-ray interacting in a solid state detector produces secondary ionizing radiation which create electron-hole pairs. The number of electron-hole pairs produced is directly proportional to the energy of the absorbed gamma or X-ray. Under the influence of the electric field existing between the electrodes of the detector the electrons and holes drift towards the positive and negative electrodes, respectively, where they are collected. The drifting electrons and holes induce signals on the electrodes which are then amplified. The resulting signals are proportional to the energy of the absorbed gamma or X-ray, and thus good spectroscopic measurements can be obtained. Solid state detectors and their associated electronics tend to be compact, require little power, and their stabilization time is small.
For very accurate spectroscopic measurements, germanium detectors (i.e. Ge(Li) ) are used (see, for example, F.S. Goulding, Nuclear Instruments and Methods, Vol.43, pp.1-54, 1966). These detectors provide very accurate energy measurements owing to the low energy required to produce an electron-hole pair, and the correspondingly large number of electron-hole pairs produced per gamma or X-ray interaction. Germanium detectors must operate at liquid nitrogen temperatures because of the very small band gap. Since they operate at liquid nitrogen temperatures, mobilities are high and charge collection efficiencies are effectively unity for both electrons and holes. Imaging systems with germanium detectors exist, but are costly because the whole imaging system needs to be at liquid nitrogen temperature.
Gamma and X-ray detectors and imaging systems have been made also with silicon. Silicon detectors are very useful for gamma and X-rays with energies less than 20 keV. However, for gamma or X-rays with energies above 20 keV, the photoelectric absorption probability is low due to Silicon's low atomic number, Z=14.
Much effort has gone to developing room temperature solid state detectors with medium to high atomic number. Some of the materials which have shown promise are: CdTe, CdZnTe, HgI.sub.2, GaAs, PbI.sub.2 (M. Cuzin, Nuclear Instruments and Methods, Vol. A253, pp. 407-417, 1987; Y. Eisen, Nuclear Instruments and Methods, Vol. A322, pp. 596-603, 1992). These materials have a high absorption probability, even for gamma rays with energy of several hundred keV. However, these materials suffer from bad to poor charge transport properties for the holes. As a result, detectors from these materials exhibit incomplete charge collection properties, whereby only a fraction of the photoelectric conversions appear in a distinct photopeak, and the rest of the events show up in a broad "incomplete" energy region. To correct for this, many techniques have been developed. One of the more popular schemes, especially for CdTe detectors, involves correlating lower charge collection with longer risetimes of the pulses. The longer risetimes indicates a deeper interaction of the gamma or X-ray in the crystal, which requires a larger fraction of the collected charge to be induced by the holes (see, for example. Y. Eisen and Y. Horowitz, Nuclear Intruments and Methods, Vol. A353, pp. 60-66, 1994). Even for obtaining very good transport properties for the electrons, a fairly stringent material selection criteria must be resorted to for good detector material. These selection requirements tend to increase the material cost for good spectroscopic detectors many-fold.
Imaging systems have been constructed with an array of individual detector elements, where each detector element forms a pixel in the imaging systems. Monolithic solid state detectors with segmented readout, usually with pad (i.e. square) segmentation, with each pad serving as a pixel in the imaging system, have also been developed. Segmentation of the readout in monolithic detectors, rather than enlarging the number of individual single element detectors, is convenient and economic in that it saves a lot of processing and machining of the detector material during production.
A lot of emphasis has been placed on developing imaging systems with room temperature solid state detectors. Recently, attention has been focused on CdZnTe as a promising material for room temperature solid state gamma and X-ray detector (see, for example, J. F. Butler, C. L. Lingren, and F. P. Doty, IEEE Trans. Nuclear Science, Vol. 39, No.4, pp. 605-609, 1992). CdZnTe has a relatively high mean atomic number of Z.about.50 as compared to Z=32 for germanium. It also has very high resistivity of .rho..about.10.sup.11 .OMEGA.-cm, and as a result very low leakage current. Low leakage current means very little noise, and insensitivity to changes in temperature (i.e. dark current).
CdZnTe detectors, despite exhibiting good charge transport properties for electrons, show fair to poor charge transport properties for holes. As a result, techniques for obtaining good spectroscopy using only the induced signals from the electrons have been developed, irrespective of the holes or depth of interaction. A scheme employing parallel grids at the anode, which is sensitive to the electron signal only, has been developed (P. Luke, Applied Phys.Lett., Vol. 65, pp. 2884-2886, 1994). This scheme requires fine segmentation of the positive electrode into strips with a small difference in bias between alternating strips. The electrons are collected on the strips with the slightly higher bias.
An approach using small segmented readout elements at the anode has also been developed (H. H. Barrett, J. D. Eskin, and H. B. Barber, Phys.Rev.Lett., Vol. 75, pp. 156-159, 1995). In this approach, the smaller the readout element size the less the sensitivity to incomplete charge collection of the holes, since the small readout elements would only feel induced charge from electrons drifting in close vicinity to the readout element. This approach obviously also benefits from the very fine spatial resolution accorded by the small readout elements. The main disadvantage of this technique is that it requires very fine segmentation of the detector, with the correspondingly high cost and complexity of large number of channels of electronics and also the corresponding slowness in readout acquisition time.
It should be mentioned that all these approaches for obtaining good spectroscopy using only the electron induced signal still require detectors with very good electron transport properties. This requirement imposes a fairly stringent selection criterion which reduces the yield of spectroscopic grade detector material and thereby significantly increases the cost of good spectroscopic detectors.
The present invention provides a method for obtaining excellent spectroscopic performance for room temperature solid state detectors. The method allows detector operation which essentially ignores the hole transport properties of the detector, and which is compatible with a wide variation in electron transport properties of the detector. Optimal implementation of the method of the present invention is obtained when using a detector with a segmented positive electrode, where each of the segmented elements serves as an individual readout element. Coarse segmentation of the detector is sufficient, and there is no need for fine segmentation.
The method of the present invention relies on a finely tuned balance between the effects of incomplete charge collection due to the nearly complete hole trapping on the one hand, and on the controlled amount of electron trapping on the other hand.